1. Field of the Invention
The present invention relates to steam turbines having integral steam chests, and, more particularly, to a method for modifying such turbines to separate the steam chest from the turbine casing.
2. Description of the Prior Art
A steam turbine for generating utility power includes, inter alia, a steam chest where high pressure steam from a boiler is collected and then admitted through apertures controlled by valves into the turbine casing, where its energy is utilized to rotate a power shaft or rotor. The steam chest is preferably located as close to the turbine as possible to minimize heat loss and pressure drops. Efficiency of the turbine increases with increasing temperature and pressure, but high pressures and temperatures involve inherent problems that turbine designers must address. Turbine casings must be exceedingly strong to withstand high steam pressures, and turbine parts and ancillary equipment subjected to high temperatures must be free to expand and contract with temperature changes. Walls thick enough to withstand the high pressures involved can experience differential thermal expansion due to temperature gradients, resulting in plastic flow and distortion of the turbine casing. Another design problem of high pressure turbines is the difficulty in providing steam-tight seals at the interface between the steam chest and the turbine casing.
There are in present use a number of turbines of a design in which the steam chest is an integral part of the turbine casing. The steam chest is typically a shell-like casting welded directly to the turbine casing. This design of turbines also typically utilizes a cam-driven system to operate the control valves in the steam chest that control admission of steam into the turbine casing.
The integral steam chest design, which incorporates a heavy mass of metal, is subject to severe thermal stresses during load cycling, and serious cracking has occurred at the weld junction between the steam chest and turbine casing. With the large integral mass of metal, a large temperature change, for example, from 1000.degree. F. to 700.degree. F., may occur across the valve seats and across the heavy shell area in a very short distance, due to valve throttling. One way to eliminate the thermal stresses in the thick mass of metal, which stresses are known to cause cracking at the chest to turbine joint, is to reduce the volume of metal at the steam chest connection point. Another proposed solution is to form a dog-bone type joint where the steam chest is welded to the casing. However, it is not believed that modification of the joint will prevent cracking due to thermal stresses unless the mass of metal is reduced to provide flexibility and freedom for differential expansion and contraction.
Another problem with the integral steam chest design that creates stress on turbine hardware is the cam-driven lift system for the control valves in the steam chest. Furthermore, the cams on the cam shaft lift the valves in a fixed sequence, which does not allow for full arc admission at start-up and may result in additional stress. This problem is thoroughly discussed in U.S. patent application Ser. No. 07/257898 filed and assigned to the Westinghouse Electric Corporation, incorporated herein by reference.
Various means of providing flexibility and freedom for thermal expansion and contraction have been discussed in the prior art. U.S. Pat. No. 1,522,191, issued Jan. 6, 1925 to Junggren, discloses a turbine design with a divided structure of parts alleged to have independent radial expansion and contraction from temperature gradients. U.S. Pat. No. 3,746,463, issued July 17, 1973 to Stock et al., proposes to reduce thermal stress with a design having, inter alia, an inner and outer casing, capable of radial movement relative to each other. U.S. Pat. No. 3,677,658, issued July 18, 1972 to Dinenno, also proposes casting the turbine in separate pieces and then bolting them together, thereby obviating the difficulties entailed in casting a complex one-piece unit, and providing a structure in which the components, including the steam chest and the nozzle chamber, have some limited ability to expand and contract relative to the other components. U.S. Pat. No. 4,697,983, issued Oct. 6, 1987 to Yamaguchi, suggests replacement of a 90.degree. nozzle box with a 180.degree. nozzle box and employs a steam inlet pipe with sliding steam-tight connections at either end and a steam-tight flange in the middle. U.S. Pat. No. 3,773,431, issued Nov. 20, 1973 to Bellati et al., discloses multiple-shell turbine casing of welded sheet metal, axially divided, alleged to keep non-uniform deformation forces of the casing away from the guide blade carrier, and has an outer casing with at least one thermally variable lead-through for a steam inlet pipe to an inner casing. U.S. Pat. No. 4,592,699, issued June 3, 1986 to Maierbacher, discloses a clamping assembly for aligning the gas ports of a pair of casings, such as a steam chest and steam turbine casing, and securing them to form a gas-tight seal between the casings which enhances distribution of pressure loads. The assembly utilizes adjustable fasteners which can effect relative lateral displacement between the casings. None of these prior inventions, however, teaches or suggests a solution to the particular problem of how to eliminate thermally generated cracks in steam turbine having integral steam chests.